The present invention relates to a voltage detection circuit for detecting an input voltage having a predetermined level by comparing the input voltage with a reference voltage, and more particularly, to a voltage detection circuit using the threshold voltage of a semiconductor active element such as a transistor as the reference voltage.
In general, a comparison circuit is employed as a voltage detection circuit for detecting input voltage, and the comparison output from the comparison circuit is used as a voltage detection output. An input voltage to be detected is applied to one input terminal of the comparison circuit, while a reference voltage obtained by using the power supply voltage or a constant voltage element (e.g., a zener diode) is supplied to the other input terminal. Such a voltage detection circuit offers an advantage that the voltage level for detecting input voltage can be made to vary by adjusting the reference voltage. However, it is hard to maintain the predetermined value of the reference voltage which relies upon the power supply voltage or a constant voltage of a constant voltage element, when the power supply voltage transiently changes at the time of the turning on or off of the power, or when the power supply voltage is so low that it becomes impossible for the constant voltage element to maintain a fixed voltage.
Instead of using the reference voltage having the mentioned drawback, a threshold voltage of a semiconductor active element such as transistor may be employed as the reference voltage to be compared with the input voltage to be detected. The threshold voltage is such a voltage that the semiconductor active element is activated by a voltage higher than that. Since the threshold voltage is fixed and independent of the power supply voltage, the aforementioned problem ocurring when the power supply voltage is in a transient state can be eliminated. By attenuating the input voltage by means of a resistance-dividing circuit, adjusting the attenuation ratio of the dividing circuit and comparing the attenuated input voltage with the threshold voltage, it is possible to vary the voltage level for detecting the input voltage. However, although the threshold voltage is constant with respect to the power supply voltage, it has a temperature coefficient. In other words, the threshold voltage varies in response to changes in the ambient temperature. Consequently, the input voltage detection level varies with temperature, and it is impossible to expect a reliable detection operation.